Residual stress in semiconductor wafers and dies causes warpage. For example, deposited materials (e.g., to create transistors) on the wafer can be engineered to have a different stress than the substrate resulting in unbalanced stress. In other cases, the stresses are not engineered but merely result from different materials. When the stress between the substrate and deposited materials is unbalanced, the substrate may warp or bend to reach an equilibrium stress.
In addition, change of temperature experienced by a packaged die can cause warpage. The coefficient of thermal expansion (CTE) of the package differs from the CTE of the die. Warpage occurs as a result of the CTE mismatch between the material sets of the package and die. The warpage is exacerbated when there is a substantial thickness difference between the package and the die.
One example of a product having a substantial thickness difference between the package and the die is a stacked IC. Thin wafers are conventionally used in stacked ICs to assist fabrication of through silicon vias. In some cases the die may be thinned to less than 50 microns without changing the thickness of a 1 mm package. As a result of the substantial thickness difference, severe warpage may occur.
When the warping is severe, inadequate bonding of the die to the package occurs. In other words, the warpage prevents some bumps or pillars from attaching to the substrate during the package assembly process. If the warpage occurs after assembly, the bumps or pillars may de-attach when an end user device is with the consumer.
As seen in FIG. 3, a warped package substrate 310 is not coupled to a warped die 320 in the center. That is, centrally located interconnects 330 do not contact the package substrate 310. Although not shown, the thermal mismatch can stress the interconnects 330 in the corner, disconnecting the package substrate 310 from the die 320.
In addition, interconnect fatigue life decreases when the coefficient of thermal expansion (CTE) between the die and package substrate are mismatched. When the temperature changes, the assembly bends to accommodate the mismatch in expansion. Based on measurements and mechanical models, warpage appears to occur at a periphery of the die, especially at the corners. The strain concentrated at the corner of chip results in a crack that propagates out from the corner. As the crack propagates, it opens up either the chip-underfill interface or another weak interface, causing either interconnect fatigue or electrical failure in the chip dielectric.
Although stress engineering solutions involving die dielectric interfaces exist, such solutions are relatively complex and expensive. Thus, a need exists for efficiently controlling warpage of a die.